The subject matter disclosed herein relates to electric machines. More specifically, the subject disclosure relates to magnetic flux regulation of permanent magnet electric machines.
Permanent magnet electric machines such as motors and generators have been in use for many years. Permanent magnet machines have been favored over other types due to their efficiency, high power density, simplicity, robustness and tolerance to large radial air gaps between the rotor and the stator of the machine.
In operation, the EMF per phase of an a.c. electrical machine is expressed by the following equation:E=π√{square root over (2)}fNkwΦ=π√{square root over (2)}npNkwΦ  (1)where f is the frequency, N is the number of turns per phase, kw is the winding factor and Φ is the main magnetic flux. The frequency is a function of the speed n in rev/s and the number of pole pairs p, i.e.:f=np  (2).
For constant magnetic flux (Φ=const) and constant number of turns per phase (N=const) the EMF (E) is a function of speed. The higher the speed, the higher the EMF and vice versa. In standard PM brushless machines there is no possibility to control the magnetic flux Φ. Thus, the faster the rotor turns, the greater the EMF induced in the stator windings of the machine.
So-called flux regulated generators/motors have magnetic flux diverters or additional windings or both. By diverting the PM magnetic flux or changing the current in a control winding, the main magnetic flux of PMs linked with the armature winding can be reduced or magnified to control the EMF. The same principle can be used in flux-weakening PM brushless motors used, e.g., in electric vehicles. When the speed is high, the magnetic flux must be reduced